With the increased use of plain paper copiers, development powders have enjoyed an increased popularity over liquid toners. Along with the increased use of development powders, magnetic brush units are becoming increasingly popular. Development powders used with magnetic brush units usually have an iron powder which serves as the carrier particle material to which toner is electrostatically attached.
While ordinarily capable of producing good quality images, conventional development powders suffer serious deficiencies in certain areas. In the reproduction of high contrast copies such as letters, tracings and the like, it is desirable to select the toner and carrier particles so that their mutual electrification is relatively large, the degree of such electrification being governed in most cases by the distance between their relative positions in the triboelectric series. However, when otherwise compatible electroscopic powder and carrier materials are removed from each other in the triboelectric serious by too great a distance, the resulting images are very faint because the attractive forces between the toner and carrier particles compete with attractive forces between the toner particles and the latent electrostatic image. Although such image density may be improved by increasing the toner concentration in the development powder, undesirably high background toner deposition as well as increased toner impaction and agglomeration is encountered when the development powder is overtoned. In some systems the initial electrostatographic photoconductor charge may be increased to improve the density of the deposited powder image, but this would have to be excessively high in order to attract the electroscopic powder away from the carrier particles. It is therefore apparent that many materials which otherwise have suitable properties for employment as carrier particles or toners are unsuitable because they possess too high a triboelectric value.
Although it may be possible to alter the triboelectric value of a carrier by blending one carrier material with another carrier material having a triboelectric value remote from the triboelectric value of the original carrier material, relatively larger quantities of additional material is necessary to alter the triboelectric value of such original carrier material. The addition of large quantities of material to the original carrier material to change the triboelectric properties thereof requires a major manufacturing operation and often undersirably alters the original physical characteristics of the carrier material. Further, it is highly desirable to control the triboelectric properties of carrier surfaces to accomodate the use of desirable toner compositions while retaining the other desirable physical characteristics of the carrier material. The alteration of the triboelectric properties by treating the surface is a desirable technique, as taught in U.S. Pat. Nos. 3,922,381 and 3,989,648. This enables one to attain the desired triboelectric properties of the carrier surface through the adhesion and/or adsorption of selected material but is limited to use where the other desired physical properties are extant.
Additionally, in magnetic-brush development of electrostatic images, for a given developer-hardware system, the carrier particles are required to have some specific system related electrical resistance to produce good quality solid area development. Some systems require a low resistivity, and to achieve this electro or electroless plating has been required as described in U.S. Pat. No. 3,736,257. Others have required the dispersion of conductive materials such as carbon black, inorganic salts, and the like as described in U.S. Pat. No. 3,533,835.
The alteration of the resistivity of a carrier particles by applying a surface coating thereon is a particularly desirable technique. Thus, for example, a carrier having the desired physical properties with the exception of electrical resistivity, can be coated with a material having the desired degree of electrical resistivity as well as other physical properties, rendering the resultant product more useful as a carrier. In order to attain the right triboelectric relationships with the desired toner, a charge-directing agent is used to enhance this characteristic. While a carrier prepared with a polymer-charge directing agent blend has utility, it also has disadvantages. Batch to batch uniformity is poor, triboelectric properities may change with time. The origin of these difficulties probably lies in the incomplete compatability of the charge-directing agent with the polymer and possibly due to the leaching of the agent from the carrier coating.
The foregoing discussion demonstrates that there is need for better electrostatographic carriers, and an improved method for making the same. From the above discussion it can be seen that it is necessary to select a carrier with proper triboelectrification for suitable toners and that electrical resistivity is an important parameter.
The coated carrier particle should also have a coating that resists impaction of the toner, resists abrasion, has good adhesion to the core material and has good flow properties. Many coating materials can be found to have some of the desired properties, no material has them all. Many polymeric materials could be synthesized but have little utility due to solubility in high boiling solvents, making coating difficult and/or costly, poor film forming properties, and high melting points making it difficut to coat via melt techniques.
It is believed that the instant invention permits greater flexibility in the design of carrier materials by ignoring triboelectric properties during the coating process, then modifying the surface of the coated particle via chemical reaction.